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8/6/2019 Perancangan Menara

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Column Design

Gavin Duffy

School of Electrical Engineering

SystemsDIT, Kevin Street


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Learning Outcomes

After this lecture you should be able to.

Explain why the ratio of vapour and liquid velocities isimportant

Describe how the vapour velocity can be calculated

Calculate the volumetric flow of vapour

Determine the cross sectional area and diameter of thecolumn


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Column Design StepsFlowrates - Carry out a mass balance to determinemass/molar flowrates of feed, distillate and bottomsand of vapour and liquid in both sections of the

columnColumn height - Determine the number of equilibriumstages. Choose a tray or packing and divide numberof equilibrium stages by tray efficiency to get actualnumber of plates or total height of packing. Also, forplates, choose the plate spacing and depthColumn diameter determine the vapour velocity

and divide vapour flowrate by velocity to give areaNow have a rough idea as to column size andnumber of trays or height of packing


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Liquid and Vapour flow Vapour flow rate can cause entrainment and flooding if too high and weeping if too low

E n t r a i n

m e n

t Entrainment FloodingD o w

n c o m

e r F l o o d i n

g Excessive Weeping

Dump P oint

Operating Region

Weep po in t

Liquid flow rate

V a p o u r

f l o w

r a

t e


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Vapour Velocity Vapour velocities are determined for both the rectifying and strippingsections of the column. They may be different.

If too low, weeping occurs liquid flows through the holes in thesieve tray, for example.If too high flooding will occur and liquid will back to the next plate.High velocity can reduce plate efficiency because the contact timebetween the phases is reduced.

The correct velocity is somewhere in between. The correct velocitydepends on the tray type.

Calculate the upper limit for velocity at the point at which flooding

occurs. A design velocity of 80 to 85% of the flooding velocity is then used(ref C&R Vol 6, 11.13).


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Flooding Velocity CalculationFor either section, the flooding velocity is estimated fromthe following equation:

v

v L f K u

= 1

Whereu f = flooding velocity m/s

K 1 = a coefficient obtained from a chartL = liquid densityv = vapour density


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Flooding Velocity contd A chart of K 1 versus F LV is available in most books ondistillation (McCabe Smith, C & R, etc.). The chart isspecific to the type of tray, e.g. sieve. The spacing

between the plates must be known. F LV is the liquidvapour flow factor and is given by:

L

v

w

w LV

V

L F

=

WhereL w = liquid mass flow rate kg/s V w = vapour mass flow rate kg/s

L = liquid densityv = vapour densitySome restrictions do apply to this chart such as minimumhole diameter, weir height, non foaming system, liquidsurface tension.


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Chart of K 1

versus FLV

From C & R, Vol VI, 3 rd Ed., p567


9/18

Vapour Density CalculationIf the vapour density is unknown, then it can becalculated using the ideal gas law:

nRT PV =Where P = column pressure

V = volume of gas (unknown)n = no. of mols, I.e. the kmol in kmol/hr or can

do on a1 mol basis

R = universal gas constant, 8.314 J/K T = temperature of vapour

Calculate V using the above equation

Then determine density by multiplying by the molecularweight and dividing by V.


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Density calculation exampleWhat is the density of air outside your airplane window?T = T o + H.k

To = Temp at ground level (25 degC)

H = height (m)k is temp gradient over height = -0.0065 degC/m

P = P o(1 + H.k/T o)gM/-Rk

Po = atmoshperic pressure

g = acceleration due to gravityM = molecular weightR = universal gas constant

From above we get P and T for air at this elevation

Now, determine the density(Should be about 0.4 kg/m 3 for an elevation of 10,000m)


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Column Diameter

The vapour flow rate in either section of the column isobtained from the mass balance in kmol/hr. This is

converted to m3 /s as follows:

m3 /s = (kmol/hr x mol wt.)/(density x 3600)

From the continuity equation, q = va. Since we know the

velocity and the flow rate we can determine the crosssectional area and from that the diameter.

Two different velocities will give two different diameters.

The same column diameter (the larger) can be used forthe entire column to simplify construction.

In this case, the plates in the lower velocity section willhave less perforations.


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Check for weeping

( )[ ]v

hh

d K u

= 4.2590.02

It is good practice to check that weeping will not occur.For weeping the vapour velocity through the holes in thetray is important.

This is obtained by dividing the minimum vapour flow rate(m 3 /s) by the area available for flow, I.e. the total holearea.

This is compared to the vapour velocity at which weepingstarts to occur which is given by:

Where d h = hole dimeter and K 2 is obtained from a chart.

The minimum velocity must be greater than the weepvelocity.


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Tray Spacing

Tray spacing determines the column height.

Plates are typically spaced 0.15 to 1m apart

If diameter is > 1m, use a plate spacing of 0.5m


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Packed ColumnsPressure drop < 1000 Pa per m height of packing(1.5 per ft in Seader & Henley, 2 nd ed., p233)

Nominal packing diameter < 1/8 th column diameter Vapour Liquid flow factor calculated as before (F LV ) Another chart is used of F LV versus Y with lines of constant pressure drop per length of packing

The Y factor contains the gas velocity we are lookingfor aswell as correction factors for gas density, liquiddensity and liquid viscosity (the chart was preparedusing data based on trials using water)This is known as the Generalised Pressure DropCorrelation, GPDC


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GPDC Chart (example only)

FLV


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Y Factor for packed column

( ) ( ) L LO H

g P o f f g

F uY

L

=

)(2

2

uo = gas velocity

FP = packing factor from a table of properties forpackings

g = gravity

g = density of gas

f( L) = liquid density correction factor

f(L) = liquid viscosity correction factor


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How to calculate column diameter

1. Calculate FLV 2. Decide on flooding pressure drop

3. Read value for Y from chart4. Rearrange eqn to get u

v,f 5. Use continuity eqn to get c.s.a. and then diameter


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Packed versus Plate Columns

For D < 0.6m packing ischeaperPacking can be made frominert/chem resistant materialso good for corrosiveGood efficiency with low Pso good for vacuumPacked column copes betterwith foaming liquidHoldup of liquid is low

Plates can be easier to cleanPacking can break moreeasilyHigh liquid rates moreeconomical in plate columnLow liquid rates less of aproblem in plate column(incomplete wetting of packing)

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